Observe all the royal guests who attended the celebration. When you see King Vallance of Asgarnia it will be apparent that he looks ill. Not yellow, but sick. Scan him, and you'll find him to be in the grip of an slug that is adept at camoflaging victims. In the kitchen, you will spot a Black Knight Assasin leaving through the window. To find out whether the fish is poisoned, examine it. Instead of throwing it out, however, have Savant apply her magic to it. Serve it to King Vallance He will fall unconscious. The slug is not dead, it's just unconscious. King Vallance was paralyzed by altered poison in the fish, and the Temple Knights have an excuse for dragging him away to remove the slug.
In the Temple Knight Base, ask the Commander Callon if you can have a look at King Vallance to see if you can find a way to remove the slug. He'll tell you that due to the slugs and Karamthalu that you gave them, they think they've got a way of cutting the slug's connection to the King. You will see the King recovering, and then ask him if he still has the slug's memories. He describes dreams of a cave at sea. He will provide Savant the coordinates of the cave.
Visit the port of Ardougne and talk to the man standing in front of the tiny sailboat to reach the cave. The man is actually a Temple Knight and the codeword to use is "Dancing to get stuff pl0x". He'll guide you to a small rock in the sea that's soaking wet. There is a cave if you head towards the west. In it, you will do an identical maze to Kennith's concerns quest. When you're finished, you'll realize that you can no longer go inside the lair. Your character will suggest Aggie to you in Draynor Village.